This invention relates to a method for preparing tubular copper or copper-alloy chills or ingot moulds of the type shaped with a substantially curved longitudinal axis, and designed for continuous steel casting plants.
In a continuous steel casting plant, said chills are traversed in known manner by a stream of fluid metal which commences to solidify during its passage therethrough, under the action of energetic cooling produced by circulating a coolant fluid which laps the outer surface of said chills.
In order to effectively perform the functions required of them, chills of this type must have a number of favorable properties. Firstly, they must be provided with internal surfaces of a high degree of hardness and with a finish such as to allow the deposition of a layer of lining material able to effectively resist the wear action deriving from the running of the molten steel, and to enable this running to take place with low friction. In addition, the chill cross-section must decrease gradually along its axis (conical profile), so as to always ensure perfect heat transfer from said surfaces to the coolant medium which laps the outer chill surface. In this respect, it has been found that if this cross-section reduction along the axis is not provided, the metal can separate from the inner chill surface due to the shrinkage of the material solidifying in the most outer layers, this considerably reducing the heat transfer coefficient between the metal and the chill itself.
Chills of the said type are normally prepared from a tubular semi-finished product with a rectilinear axis, formed by simple extrusion or by any other operation. It is then given a curved shape, normally by exerting radial pressures on its outer surface using a mold of suitable form. Then in order to create the required surface finish and the cross-sectional variation along its axis, as is necessary to obtain correct flow of steel along the chill, said surface is machined by millers or grinders of special type which are moved along inside the semi-finished product by means of devices of special shape. In an alternative method, the cross-sectional variation inside the chill is obtained by chemical attack using a suitable chemical agent with which the cavity inside the chill is filled. By decreasing the level of said liquid proportionally to the time, the surface is chemically attacked, resulting in the removal of quantities of material which are proportional to the axial length of the chill. The chills obtained by the aforesaid methods have numerous drawbacks. Firstly, the hardness of the inner surface of the chill is very low and substantially equal to that of the material of the initial semi-finished product. In addition, its surface finish is also not particularly good, especially if subjected to the aforesaid chemical action. Again, the required inner shape of the chill can be obtained only with a certain approximation, this applying particularly to the variation in the inner cross-section along the axis. Finally, the necessary machining in order to prepare chills by the firstly described method can be particularly lengthy, difficult to carry out and generally require special care.